(a). Field of the Invention
The present invention relates to a front derailleur for a bicycle, and more specifically to a front derailleur for a bicycle that can achieve efficiency and stability during a shifting operation.
(b). Description of the Prior Arts
A bicycle is commonly used as a transportation vehicle; and widely employed as a sport machine. Basically, the bicycle includes several parts, such as transmission system, brake system, frame, and so on, although these parts vary from different types of bicycles. The frame constructs the main body of a bicycle. The transmission system which is a mechanism in combination of chain, chainwheel, freewheel, pedal, and soon, plays a role to transmit a force generated from cyclist to drive the bicycle. In the derailleur system for a bicycle A, as shown in FIG. 1, which includes a seat tube A1, a chainwheel B1, a freewheel B2, a chain C, a front derailleur D1, a rear derailleur D2, a rocking arm D3, a wire (not shown), and a shift lever (not shown), the chain C can be shifted among sprockets B11 of the chainwheel B1 and sprockets B21 of the freewheel B2 by the front derailleur D1 and rear derailleur D2 in response to force generated by cyclist or return spring of derailleur system. Since the sprockets of the chainwheel B1 and the freewheel B2 have different teeth, the cyclist may adjust the chain through the front derailleur and the rear derailleur to obtain a gear ratio between the sprockets of the chainwheel B1 and the freewheel B2, so as to ride the bicycle A in a time-saving or labor-saving mode. Therefore, the derailleur system is a vital factor for the performance of bicycle.
Front derailleurs for bicycles are well known in the art. Illustrative are the front derailleurs disclosed in U.S. Pat. Nos. 5,496,222 and 5,620,384. The typical front derailleur includes a fixed bicycle frame member, a chain guide having an outer guide plate and an inner guide plate which are disposed at opposite sides of the chain C, and linkage members comprising an actuating arm and a linkage rod. The linkage members are connected to the fixed bicycle frame member and the chain guide through pivot pins extending perpendicularly to the axis of the multistage front sprockets to thereby form a parallelogram linkage mechanism which allows the chain guide to move axially of the front sprockets.
The parallelogram linkage mechanism usually is provided with a return spring which biases the chain guide toward the bicycle frame, that is, from the larger diameter front sprocket to the smaller diameter sprocket. In operation, when a cyclist pivots the shift lever to pull the control cable against the return spring to actuate the linkage mechanism which moves the chain guide axially of the multistage front sprockets, thereby shifting the chain from a smaller diameter sprocket to a larger diameter sprocket. To shift the chain from the larger diameter sprocket to a smaller diameter, the control cable is loosened and the restoring force of the return spring moves the chain accordingly.
The chain guide of the linkage mechanism, as disclosed in U.S. Pat. Nos. 5,496,222 and 5,620,384, is operable through a pivoting range between the larger wheel side tilt position and the small wheel side tilt position. The imaginary vertical line V accordingly is located within the pivoting range, as shown in FIG. 4, where the return spring applies a turning force PL to the chain guide when the chain guide is in the low speed position. An operating force FL is needed to cause the chain guide to pivot from the low speed position against the return spring. When the chain guide pivots through the tilt angle A to the vertically extending intermediate position, the chain guide receives a turning force P from the return spring. An operating force F is needed to cause the chain guide to pivot from the intermediate position against the return spring. When the chain guide pivots from the intermediate position through the tilt angle B to the high speed position, the chain guide receives a turning force PT from the return spring. An operating force FT is needed to cause the chain guide to pivot to the high speed position against the return spring. Further, in FIG. 4, because the return spring undergoes a varying elastic deformation with the pivotal movement of the chain guide, the turning force P in the intermediate position becomes greater than the turning force. PL in the low speed position, and the turning force PT in the high position becomes greater than the turning force P in the intermediate position. It will become clear from FIG. 4 that FL=PL/cos E, FT=PT/cos G and angle E equals angle A, angle G equals angle B.
The aforementioned linkage mechanism moves from the left of the imaginary line V to its right. Thus, while in high spend mode (that is, the chain engages the larger chainwheel sprocket and the smaller freewheel sprocket that a greater ratio will result to enable one to ride the bicycle at high speed), the loading of the chain is at its maximum so that the flexibility of the chain thereby is very low, and also the linkage rod is not at a near perpendicular state to be able to utilize the restoring force of the return spring filly. Besides, the inter-pivot distance Lh (as seen in FIG. 5) for the outer guide plate to act on the chain is very short. Consequently, a larger operating force is needed to translate the chain laterally of the bicycle for enabling the speed change. On the other hand, the inter-pivot distance Lm is longer than Lh during the change of speed from middle to low. For that reason, the down-shifting efficiency of the prior art in high speed mode (translating the chain from the larger sprocket to the smaller sprocket) is not preferable. To improve the down-shifting efficiency in high speed mode, the return spring will need to be strengthen and in consequence cause the cyclist more strength to operate while up-shifting.
In view of the above, there exists a need for a bicycle front derailleur which overcomes the above mention problems in the prior art. The present invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.